1. Field of the Invention
The present invention relates to pistons for internal combustion engines, and in particular to the shape of the lip of the combustion bowl of such pistons.
2. Description of the Related Art
Future emissions regulations require significant reductions in internal combustion engine emission of particulates, visible smoke, hydrocarbons and NO.sub.x. It has long been known in the industry that making the combustion bowl lip sharp or as nearly sharp as possible helps reduce such emissions. Unfortunately, the sharper the lip, the higher the mechanical stress in the lip, so this has not been a satisfactory emissions control technique in the real world.
Such stress is particularly a problem in the portion of the combustion bowl lip directly above the pin bore of the piston. The piston is thinnest there, and the stress caused by a small, e.g., less than about 1 mm nominal radius, lip is more than the piston can withstand. It typically cracks after a moderate amount of use, leading to catastrophic failure.
One possible solution would be to make the piston out of materials which can withstand a higher degree of stress than the materials conventionally used. In large engines, the pistons normally are made of cast aluminum for reduced mass. Fiber reinforced squeeze castings and ferrous one-piece or articulated pistons would be significantly stronger. Unfortunately, they also are significantly more expensive and heavier.